FG-Rich Repeats of Nuclear Pore Proteins Form a Three-Dimensional Meshwork with Hydrogel-Like Properties

Abstract

Nuclear pore complexes permit rapid passage of cargoes bound to nuclear transport receptors, but otherwise suppress nucleocytoplasmic fluxes of inert macromolecules >/=30 kilodaltons. To explain this selectivity, a sieve structure of the permeability barrier has been proposed that is created through reversible cross-linking between Phe and Gly (FG)-rich nucleoporin repeats. According to this model, nuclear transport receptors overcome the size limit of the sieve and catalyze their own nuclear pore-passage by a competitive disruption of adjacent inter-repeat contacts, which transiently opens adjoining meshes. Here, we found that phenylalanine-mediated inter-repeat interactions indeed cross-link FG-repeat domains into elastic and reversible hydrogels. Furthermore, we obtained evidence that such hydrogel formation is required for viability in yeast.

Converging on the function of intrinsically disordered nucleoporins in the nuclear pore complex

“…Upon closer inspection, it would appear that Nsp1 has the highest likelihood of forming a meshwork having a large hydrodynamic radius and being closely spaced with 32 copies. However, conflicting experimental evidence showing that yeast Nsp1 exhibits both cohesive (Frey et al, 2006;Gorlich, 2007, 2009) and non-cohesive properties (Patel et al, 2007) makes it even more difficult to predict if it forms a meshwork in the NPC.…”

“…They subsequently showed and explained that mild apolar solvents (e.g., hexanediol) could cause a reversible collapse in the FG-domain barrier by perturbing the hydrophobic inter-FG interactions (Ribbeck and Gorlich, 2002). Subsequently through extensive investigations, Görlich and co-workers have since shown that FG-domains can form crosslinked macroscopic hydrogels from solution (Frey et al, 2006;Gorlich, 2007, 2009). Interestingly, Frey et al showed hydrogel formation for both FG-/FxFG-(Nsp1) and GLFGdomains (Nup49 and Nup57) (Frey and Gorlich, 2009).…”

“…By correlating these properties to their estimated localizations in the NPC, the 'two-gate' model (Patel et al, 2007) proposes that the central channel is occupied by a FG-hydrogel whereas the FGdomains at the peripheries are brush-like. However, conflicting reports remain as to the observed non-cohesive properties of the FxFG domain Nsp1 in comparison with the hydrogelforming property of Nsp1 (Frey et al, 2006;Gorlich, 2007, 2009). More generally, the class of GLFG domains that was postulated to favor hydrogel formation is particularly lacking in vertebrate NPCs and is only found in yeast (Table 2).…”

“…In the ensuing competition between the different interactions, the central channel would be occluded by the FG-domain meshwork if the degrees of attractive 'cohesive' interactions dominate locally over the repulsive ones. A consideration of these contextual details might explain why the equivalence of this scenario and that of macroscopic gels constructed from non-tethered FG-domains in solution (Frey et al, 2006;Gorlich, 2007, 2009) is not obvious.…”

“…In spite of their variety, these models differ only in the physical arrangement and mobility (static vs. dynamic) of the FGdomains within the NPC. So far, studies targeting the materials properties of the FG-domains directly show that FG-domain behavior can be classified as resembling (i) a polymer brush (Lim et al, 2006(Lim et al, , 2007aMiao and Schulten, 2009) (i.e., dynamic), (ii) a bulk-like hydrogel meshwork (Frey et al, 2006;Gorlich, 2007, 2009) (i.e., static) or (iii) both (Patel et al, 2007). Here, we outline the differences between these characteristics and will attempt to address their convergent properties later on.…”